feat(backend): restructure API into layers with OpenAPI, versioning and error handling

Split the backend into API host, Domain, Application and Infrastructure projects. Add Swagger/OpenAPI per API version, Asp.Versioning (query/header), ProblemDetails with a global exception handler and SurrealQL migrations applied at startup (namespace/database ensured). Infrastructure owns the SurrealDB client, settings and DI.

backend/TarninaHub.csproj

@@ -9,9 +9,30 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="10.0.12" />
-    <PackageReference Include="SurrealDb.Embedded.RocksDb" Version="1.0.0" />
-    <PackageReference Include="SurrealDb.Net" Version="1.0.0" />
+    <!-- Layered projects are nested under backend/ for deployment; exclude their
+         sources from the API host compile (they are referenced as projects). -->
+    <Compile Remove="Application/**/*.cs" />
+    <Compile Remove="Domain/**/*.cs" />
+    <Compile Remove="Infrastructure/**/*.cs" />
+    <Content Remove="Application/**" />
+    <Content Remove="Domain/**" />
+    <Content Remove="Infrastructure/**" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Content Include="migrations/**/*.surql" CopyToOutputDirectory="PreserveNewest" CopyToPublishDirectory="PreserveNewest" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Asp.Versioning.Mvc" Version="10.2.1" />
+    <PackageReference Include="Asp.Versioning.Mvc.ApiExplorer" Version="10.2.1" />
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="10.2.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="Application/TarninaHub.Application.csproj" />
+    <ProjectReference Include="Domain/TarninaHub.Domain.csproj" />
+    <ProjectReference Include="Infrastructure/TarninaHub.Infrastructure.csproj" />
   </ItemGroup>
 
 </Project>